Redundant imaging methods and systems

ABSTRACT

Imaging arrays typically include thousands or millions of photodetectors that convert sensed light into corresponding electric signals, which are ultimately converted into digital image signals for recording or viewing. One problem with conventional imaging arrays concerns faulty photodetectors, which produce erroneous image signals that ultimately degrade the quality of resulting images. Accordingly, the present inventors devised new imaging arrays including redundant photodetectors to compensate for faulty ones. One exemplary embodiment includes photodetectors that are substantially smaller than conventional photodetectors and that are arranged into two or more groups, with the photodetectors in each group coupled to produce a single group image signal. If the group image signal for a group falls below some threshold level indicative of a defective or malfunctioning photodetector, the group image signal is amplified to compensate for the loss.

[0001] This application is a Divisional of U.S. application Ser. No.09/650,551, which was filed Aug. 30, 2000 and which is incorporatedherein by reference.

TECHNICAL FIELD

[0002] The present invention concerns imaging arrays and methods,particularly methods for correcting or compensating for defective ormalfunctioning photodetectors in an imaging array.

BACKGROUND OF THE INVENTION

[0003] Imaging arrays are electronic devices that sense light and outputelectrical signals representative of the sensed light. The imagingarrays are generally coupled to a television screen, computer monitor,or digital camera, which displays or records an image based on theoutput electrical signals.

[0004] An imaging array often includes a rectangular array or matrix ofthousands or even millions of photodetectors, with each photodetectorhaving a unique row and column position within the array whichcorresponds to a particular region, known as a pixel, of a displayedimage. Each photodetector (or sensor pixel) converts sensed light intocorresponding electric signals based on the intensity of the light. Theelectrical signals are converted into digital signals, comprising onesand zeros, which are processed by a digital-signal-processing circuit.This circuit ultimately outputs image signals to a device for recordingor viewing.

[0005] One problem with conventional imaging arrays concerns defectiveor malfunctioning photodetectors. Defective photodetectors typicallyresult in erroneous image signals that ultimately degrade the quality ofresulting images. For example, an image based on imaging signals from animaging array having a defective photodetector can have a black or darkarea at the image region corresponding to the defective photodetector.

[0006] One limited solution to this problem has been to identify thedefective photodetector and to generate a substitute image signal forthe image signal of the defective photodetector, with the substituteimage signal based on an average of the image signals from detectorssurrounding it. See, for example, U.S. Pat. No. 5,854,655 (which isincorporated herein by reference). However, this solution suffers fromthe disadvantage that the substitute image signal introduces artifactsinto the resulting image. The artifacts reflect the complete loss ofinformation about the light actually striking the relatively large areacorresponding to the defective photodetector.

[0007] Accordingly, there is a need for other methods of handlingdefective photodetectors.

SUMMARY OF INVENTION

[0008] To address this and other problems, the present inventor devisednew imaging arrays and related methods for compensating for defectivephotodetectors. One exemplary embodiment of a new imaging array includestwo or more group photodetectors, or “group pixels,” with each grouppixel having two or more photodetectors coupled to produce a singlegroup image signal. If the group image signal for a group pixel fallsbelow some threshold level indicative of a defective or malfunctioningphotodetector, the group image signal is amplified to compensate for theloss.

[0009] Various embodiments implement the photodetectors as passive oractive photodiode circuits, as photogate circuits, as logarithmic sensorpixel circuits, or as charge-modulation devices. Some embodiments alsoimplement the photodetectors as smaller-than-conventionalphotodetectors, that is, photodetectors having photosensing elementssmaller than conventional elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a block diagram of an exemplary imaging array 100incorporating the invention.

[0011]FIG. 2 is a block diagram of an exemplary group-pixel circuit 200incorporating the present invention.

[0012]FIG. 3 is a block diagram of an exemplary pixel circuit 300.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The following detailed description, which references andincorporates FIGS. 1-3, describes and illustrates one or more specificembodiments of the invention. These embodiments, offered not to limitbut only to exemplify and teach, are shown and described in sufficientdetail to enable those skilled in the art to implement or practice theinvention. Thus, where appropriate to avoid obscuring the invention, thedescription may omit certain information known to those of skill in theart.

[0014]FIG. 1 shows an exemplary imaging array 100 incorporatingteachings of the present invention. Imaging array 100 includes grouppixels 110, 112, 114, and 116, an address line 120, a drain line 130, areset line 140, and a signal line 150. for controlling the group pixels.(For clarity, the figure omits conventional features, such as row-selectlogic, column-select logic, timing-and-control circuitry, andanalog-to-digital converters.) In the exemplary embodiment, array 100includes four group pixels; however, other embodiments include 256×256arrays, 512×512 arrays, 1024×1024 arrays. Still larger arrays are alsowithin the scope of the invention.

[0015] Each of group pixels 110-116 includes two or more photodetectors,or sensor pixels. Group pixel 110 includes sensor pixels 110 a, 110 b,110 c, and 110 d, and group pixels 112, 114, and 116 include respectivesensor pixels 112 a-112 d, 114 a-114 d, and 116 a-116 d. Lines 120, 130,and 140, in the exemplary embodiment, control the group pixel in accordwith known techniques for addressing and controlling conventional sensorpixels in imaging arrays. In some embodiments, each group pixelsprovides a particular output color, such as red, blue, or green.

[0016]FIG. 2 shows a block diagram of an exemplary group-pixel circuit200 applicable to each of group pixels 110-116 in FIG. 1. Circuit 200includes N sensor pixels, of which sensor pixels 202, 204, 206, and 208are representative, a summer 210, a variable-gain amplifier 212, and anautomatic gain controller 214. The N pixels 202-206, which operateaccording to known principles, are coupled to an input of summer 208,either through direct connection or through a multiplexer (not shown).Some embodiments include one or more analog-to-digital converterscoupled between the signal lines of the pixels and the summer, dependingon whether summer 210 is analog or digital.

[0017] Summer 210 aggregates the N responses of the N pixels 202-206 andoutputs a first aggregate or group image signal to amplifier 212. (Someembodiments include in analog-to-digital converter between the summerand the amplifier.) Amplifier 212, which in some embodiments is analogand in others is digital, amplifies or scales the first group imagesignal and outputs a second group image signal to automatic gaincontroller 214 as well as to conventional imaging processing and displaycircuitry (not shown.) See U.S. Pat. No. 5,854,655, which isincorporated herein by reference.

[0018] Automatic gain controller 214, which is analog or digital,compares the second group image signal to an analog or digital referencecurrent or voltage. If the comparison indicates that the second groupimage signal differs from the reference, controller 212 proportionatelychanges, that is, increases or decreases, the gain of amplifier 210,assuming that one or more of the N pixels or related interconnectivecircuitry is faulty. In the exemplary embodiment, gain controller 214sets the gain to a factor proportional to the ratio of N, the number ofpixels comprising the group pixel to M, the number of correctlyoperating or non-faulty pixels in the group pixel.

[0019] To determine the number of non-faulty pixels, some embodiments,check the performance of each pixel in each group pixel as a start-updiagnostic test and maintain a record of the number of faulty ornon-faulty pixels in each group pixel. Other embodiments dynamically orperiodically determine a difference between the first aggregate imagesignal and a reference, and then determine from the difference how manypixels are faulty. The reference in some embodiments is based on afactory test image.

[0020]FIG. 3 shows an exemplary sensor pixel circuit 300 applicable toeach of the pixels in FIGS. 1 and 2. Circuit 300, a photodiode-typeactive sensor pixel circuit, includes photodiode 310, a source-followerfield-effect transistor SF, a row-select field-effect transistor SL, anda charge-reset field-effect transistor RS. (An n-channel load transistorfor source-follower transistor SF is not shown.) Each field-effecttransistor has respective gate, drain, and source nodes. The circuitfurther includes an address line 320, a drain line 330, a reset line340, and a signal line 350.

[0021] In operation, a voltage develops across photodiode 310 based onincident light. Application of appropriate control signals on the gateof transistor SL produces an image signal on signal line 350 based onthe voltage across the photodiode. Signal line 350 couples the imagesignal to an input node of an analog-to-digital converter or summer,such as summer 210 in FIG. 2.

[0022] Various embodiments implement the photodetectors as passive oractive photodiode circuits, as photogate circuits, as logarithmic sensorpixel circuits, or as charge-modulation devices. (See, for example, EricR. Fossum, CMOS Image Sensors: Electronic Camera-On-A-Chip, 1995International Electron Devices Meeting Digest of Technical Papers, whichis incorporated herein by reference.) Some embodiments eachphotodetector occupies a surface area less than 30 square microns, suchas 15 or 25 square microns. Some of these embodiments have a fill factorgreater than 30 percent. Thus, the present invention is not limited toany particular photodetector circuit or class of photodetector circuits.

Conclusion

[0023] In furtherance of the art, the inventors have presented newimaging arrays and related methods for compensating for defectivephotodetectors. One exemplary embodiment of a new imaging array includesone or more group pixel circuits, each of which comprises two or morephotodetectors that are substantially smaller than conventionalphotodetectors, for example about 15 or 25 square microns. Each grouppixel circuit produces a single group image signal. The group imagesignal is then scaled or amplified to compensate for defective ormalfunctioning photodetectors.

[0024] The embodiments described above are intended only to illustrateand teach one or more ways of practicing or implementing the presentinvention, not to restrict its breadth or scope. The scope of theinvention intended to encompass all ways of practicing or implementingthe principles of the invention, is defined only by the following claimsand their equivalents.

1. An imaging system comprising: a group pixel comprising two or morephotodetectors for providing two or more corresponding pixel imagesignals; and a summer coupled to each of the two or more photodetectorsfor outputting an aggregate image signal based on the two or morecorresponding pixel image signals.
 2. The imaging system of claim 1wherein the summer comprises an analog-to-digital converter.
 3. Theimaging system of claim 1, further comprising an address line and asignal line, with each photodetector coupled to the address line and thesignal line.
 4. The imaging system of claim 1, wherein eachphotodetector is adapted to detect one of red, green, and blue light. 5.The imaging system of claim 1, further comprising a variable-gainamplifier responsive to the aggregate image signal for outputting anamplified aggregate image signal based on a gain, with the gain based ona number of faulty photodetector circuits in the group pixel.
 6. Theimaging system of claim 5, wherein the gain is based on a ratio of atotal number of photodetector circuits in the group pixel to a totalnumber of faulty photodetector circuits in the group pixels.
 7. Animaging system comprising: a group pixel comprising two or morephotodetector circuits for providing two or more corresponding pixelimage signals, with each photodetector circuit having a photodiode andoccupying a surface area less than 50 square microns; and a summercoupled to each of the two or more photodetector circuits for outputtingan aggregate image signal based on the two or more corresponding pixelimage signals.
 8. The imaging system of claim 7 wherein the summercomprises an analog-to-digital converter.
 9. The imaging system of claim7, further comprising an address line and a signal line, with eachphotodetector of the group pixel coupled to the address line and thesignal line.
 10. The imaging system of claim 7, wherein eachphotodetector is adapted to detect one of red, green, and blue light.11. The imaging system of claim 7, further comprising a variable-gainamplifier responsive to the aggregate image signal for outputting anamplified aggregate image signal based on a gain, with the gain based ona ratio of a total number of photodetector circuits in the group pixelto a total number of faulty photodetector circuits in the group pixels.12. An imaging system comprising: a group pixel comprising two or morephotodetector circuits for providing two or more corresponding pixelimage signals, with each photodetector circuit occupying a surface arealess than 30 square microns and comprising: a source-follower transistorhave a gate, source, and drain; a ground node; and a photodiode coupledbetween the gate of the source-follower transistor and the ground node;and a summer coupled to each of the two or more photodetectors foroutputting an aggregate image signal based on the two or morecorresponding pixel image signals.
 13. The imaging system of claim 12wherein the summer comprises an analog-to-digital converter.
 14. Theimaging system of claim 12, further comprising an address line and asignal line, with each photodetector circuit of the group pixel coupledto the address line and the signal line.
 15. The imaging system of claim14, further comprising a variable-gain amplifier responsive to theaggregate image signal for outputting an amplified aggregate imagesignal based on a gain, with the gain based on a number of faultyphotodetector circuits in the group pixel and each photodetector adaptedto detect one of red, green, and blue light.
 16. The imaging system ofclaim 15, wherein the gain is based on a ratio of a total number ofphotodetector circuits in the group pixel to a total number of faultyphotodetector circuits in the group pixels.
 17. An imaging systemcomprising: a group pixel comprising two or more photodetector circuitsfor providing two or more corresponding pixel image signals, whereineach photodetector circuit has a surface area less than 50 squaremicrons, is adapted to detect the same color of light, and comprises: asource-follower transistor have a gate, source, and drain; a groundnode; and a photodiode coupled between the gate of the source-followertransistor and the ground node; a summer coupled to each of the two ormore photodetector circuits for outputting an aggregate image signalbased on the two or more corresponding pixel image signals; and avariable-gain amplifier responsive to the aggregate image signal foroutputting an amplified aggregate image signal based on a gain, with thegain based on a number of faulty photodetector circuits in the grouppixel.
 18. The imaging system of claim 17, wherein the gain is based ona ratio of a total number of photodetector circuits in the group pixelto a total number of faulty photodetector circuits in the group pixels.19. The imaging system of claim 17 wherein the summer comprises ananalog-to-digital converter.
 20. The imaging system of claim 17, furthercomprising an address line and a signal line, with each photodetectorcircuit of the group pixel coupled to the address line and the signalline.